PROJECT SUMMARY/ABSTRACT Currently-available drug treatments for Alzheimer's disease (AD) are not curative. Furthermore, findings from clinical trials testing novel disease-modifying therapeutics have been disappointing. Accordingly, the urgency of alternative approaches for halting the global crisis posed by AD cannot be overstated. Although data from cohort and epidemiological studies have long suggested a strong link between physical activity and dementia due to AD, the question of whether physical activity modulates the underlying pathophysiology of AD has only recently begun receiving attention. While the emerging evidence appears overall supportive of such a role for physical activity, several critical knowledge gaps persist. First, past studies have been largely cross-sectional. This leaves unresolved the possibility that observed effects simply reflect reverse causation. Second, ?physical activity? has been assessed via a variety of approaches including self-report, activity trackers, and maximal graded exercise testing, leading to conflicting findings. Third, because past research has primarily been done in elderly persons, little is known about the potential influence of physical activity on AD risk in midlife, when most AD-related changes begin. Lastly, there is need for a better understanding of the mechanisms by which physical activity exerts its salutary effects. To address these gaps in knowledge (1) we focus this project on cardiorespiratory fitness (CRF), which constitutes the physiological nexus for *habitual* physical activity, (2) we employ a longitudinal design, which would allow us rigorously exclude the possibility of reverse causation, (3) we study a cohort of late-middle-aged adults who are, in principle, potentially only at the inceptive stages of AD, and (4) we investigate vascular and glucoregulatory function as viable transducers of the link between CRF and AD pathophysiology. Importantly, because the participants targeted for this study are being followed longitudinally through the Wisconsin Registry for Alzheimer's Prevention and the Wisconsin Alzheimer's Disease Research Center, we will be uniquely positioned in the long term to elucidate the impact of midlife CRF on clinical endpoints of mild cognitive impairment and dementia. In sum, the multimodal and integrative study proposed here stands to provide critical insights into CRF as a potentially viable therapeutic for altering disease trajectory in the early stages of AD, prior to pervasive neurodegeneration, thereby delaying the emergence of clinical symptoms.